Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a substrate or wafer. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. As design rules and process windows continue to shrink in size, inspection systems are required to capture a wider range of physical defects on wafer surfaces while maintaining high throughput.
A variety of calibration procedures are performed on semiconductor inspection systems to ensure the measurement accuracy of a particular inspection system. In some examples, a reference wafer (or set of reference wafers) with known properties is measured by an inspection system. Parameters of the inspection system are tuned such that the measurement result generated by the inspection system matches the known characteristics of the reference wafer. This procedure ensures the measurement accuracy of the calibrated inspection system for wafers with properties that closely match those of the reference wafer.
In a manufacturing environment, measurement consistency across a fleet of inspection systems tasked with the same measurement objective is also important. If measurement consistency among inspection systems degrades, consistency among processed semiconductor wafers is lost and yield drops to unacceptable levels. To some extent measurement consistency may be obtained by calibrating each inspection system of a fleet of inspection systems with a reference wafer (or set of reference wafers). However, to obtain high accuracy results, calibration experiments involving the reference wafer must be performed in a carefully controlled environment that matches the environmental conditions in place when the reference wafer was originally characterized. This may be difficult to achieve in a manufacturing environment and lead to loss of consistency among inspection systems. In addition, an expensive reference wafer set must be maintained in the manufacturing environment. Risks of wafer breakage or degradation potentially jeopardize the integrity of the calibration process. Moreover, the accuracy of an inspection system calibrated based on reference wafers is typically limited to wafers with properties that closely match those of the reference wafer.
Accordingly, it would be advantageous to develop improved calibration methods for inspection systems to ensure measurement consistency over a wide range of wafers and across a fleet of inspection systems.